Check the ringbuffer pointers for invalid values.
Check the ringbuffer pointers for invalid values.

If a logbook entry contains an invalid ringbuffer pointer, the error is
not returned immediately, but delayed until the end of the download.
With this approach we can download at least the dives before the
problematic logbook entry.

venkateshshukla/libdivecomputer,josh-wambua/libdivecomputer,venkateshshukla/libdivecomputer,Poltsi/libdivecomputer-vms,glance-/libdivecomputer,henrik242/libdivecomputer,josh-wambua/libdivecomputer,henrik242/libdivecomputer,glance-/libdivecomputer